Scanning electron microscopes and electric beam inspection devices use an electrostatic chuck (ESC) to support wafers that are located within a vacuumed chamber.
FIG. 1 illustrates a conventional chuck 60 and a wafer 10. Chuck 60 includes an isolating substrate 40, two electrodes 31 and 32 that are connected to two voltage suppliers 51 and 52, and multiple microscopic protrusions 20 that contact the wafer 10.
The chuck 60 also has one or more grounding elements (not shown) for connecting the wafer 10 to electrical ground (zero potential).
The isolating substrate 40 is made of isolating materials such as ceramics (including alumina).
The two electrodes 31 and 32 are located within the isolating substrate 40 and are surrounded by isolating material. In some cases (as illustrated in FIG. 1) an upper portion 41 of the isolating substrate 40 is made of different material than a lower portion 42 of the isolating substrate 40. The electrodes 31 and 32 may virtually separate the isolating substrate 40 to the upper portion 41 and to the lower portion 42.
Typically, one electrode 31 is connected to a positive potential (51) and the other electrode 32 is connected to a negative potential (52).
The voltage differences between the wafer 10 and each of the electrodes 31 and 32 attract the wafer 10 towards the chuck 60.
It is noted that this figure (as well as the other figures of this application) are not to scale. For example, while FIG. 1 illustrates distance d2 as being about five times bigger than distance d1, the actual ratio of d1:d2 may exceed fifty. The distance d1 between the lower surface of the wafer 10 and the upper surface of the insulating substrate 40 may be approximately 3-4 microns, while distance d2 between the lower surface of the wafer 10 and the electrodes 31 and 32 may be approximately 200 microns.
The isolating substrate 40 is made of isolating material and it accumulates charge. The charge can be developed by migration from the electrodes 31, 32, by touching the wafer 10, or by wafer 10 discharging during unloading. The charge is usually developed in proximity to the upper surface of the isolating substrate 40 and can be concentrated near the edges of the isolating substrate 40.
This charge can damage ESC performance. Residual charge can attract the wafer 10 to the chuck 60 and can prevent the wafer 10 from being easily detached from the electrostatic chuck 60 during an unload process. The charge can even cause the wafer 10 to break during the unload process.